266 DR. H. We MARETT TIMS ON 
but the enamel appears to be deficient on the posterior surfaces. 
No trace of any downgrowth, lingual or labial, is visible. Some 
distance from the anterior end of the jaw, in a position posterior 
to where the permanent incisor cuts the gum, the dental lamina 
grows deeply into the substance of the jaw; it persists through 
some sections and then ceases, it is bilaterally symmetrical. 
Possibly this may mark the position of 7.2, which has dis- 
appeared. 
Throughout the diastema no trace of any tooth-germs is visible. 
The most anterior cheek-tooth is the so-called ppm.1, it is in 
a somewhat rudimentary condition and as yet quite uncalcified. 
It consists of a single cone with a blunted summit. The internal 
cingulum is present, and also indications of an external. I 
would here specially note the presence of a spherical body, 
composed of concentrically arranged cells, lying in the line 
of the connecting neck of dental lamina (Pl. 26. fig. 9). I 
have already figured [22] a precisely similar structure in con- 
nection with pm.4 of the dog, the only difference being a slight 
one of position. In the latter case, it lies at the free extremity 
of a labial downgrowth of the dental lamina; whereas in the 
present instance, it is directly in the line of the dental lamina 
running between the oral epithelium and the tooth. In the 
guinea-pig this structure is present on both sides of the upper 
jaw, and persists for some time. A similar structure is also to 
be found in connection with the so-called m.2. I have already 
referred (doc. cit.) to the fact that Mr. M. F. Woodward has found. 
a similar structure in Gymnura in the same position, viz., in 
connection with the posterior premolar. 
In discussing the question, I there stated that I was not able to 
give an explanation of the condition, “but from the fact of its 
connection with the dental lamina and its presence in precisely 
the same situation in these forms” (i.e. Canis and Gymnura) 
“ T do not think it is a chance structure, and it is possible that it 
may represent the remains of a predecessor to this tooth.” 
From a further consideration of this point in Cavia, I am still 
more inclined to this opinion. Mr. Woodward has figured a 
calcified vestigial incisor in the mouse [25] which, as he describes, 
appears in section “as a narrow loop forming about 2 of a 
circle, a few of the mesoblast-cells having flattened themselves 
against the outer surface of the dentine.” From a comparison 
